The manufacture of glass plates generally involves the cutting and sorting of glass plates out of a glass strip pulled up from molten glass bath in the furnace as the glass strip travels on a line conveyor at a constant speed. In cutting the glass strip, trimming lines are marked in the direction parallel with the strip travelling direction by means of a trimming-line marker, cutting lines are marked in the direction perpendicular to the strip travelling direction by means of a diagonal cutter, and then the strip is cut along the cutting lines by means of a breaker and along the trimming lines by means of a slitter. In cutting the glass strip into a glass plate, various flaws are often caused on the cut edges of the glass plate, including chips produced by gouging the glass plate in a fish-scale shape in the through-thickness direction of the glass plate, serrations produced by gouging the glass plate in the through-thickness direction, burrs outwardly protruding from the cut line of the glass plate, and notches inwardly notched from the cut line of the glass plate.
Glass plates having such so-called cut-edge flaws have to be discarded as rejects. Conventional methods for detecting cut-edge flaws include the following.
First comes detection by visual inspection. This involves throwing a light beam from under a glass plate travelling on a line conveyor and monitoring by visual inspection the shadow of the glass plate projected on a screen located above the glass plate. With this method, such flaws as chips and serrations can be easily detected as shadows because light transmission remarkably deteriorated at chips and serrations, resulting in remarkably lowered transmitted light volume, while burrs and notches, which cause less changes in the volume of transmitted light, do not appear as clearly discernible shadows, making it difficult to detect them. In addition, visual inspection by human eyes has its limitation in terms of detecting accuracy.
The prior art as shown in FIG. 1 is a cut-edge flaw detecting device which does not rely on visual inspection has such a construction that a fluorescent lamp 2 is disposed below a line conveyor carrying a glass plate 1. The fluorescent lamp 2 is positioned parallel with the plane of the glass plate 1 and perpendicular to the travelling direction of the glass plate 1, a plurality of 1024-bit one-dimensional CCD (charge-coupled device) cameras 3 are disposed above the line conveyor to receive the light emitted from the fluorescent lamp 2 and transmitted through the glass plate 1. The outputs of the cameras 3 being sent to a discriminator 4 incorporating a microcomputer for processing to discriminate flaws present in the cut edges of the glass plate 1.
In the following description, the travelling direction of a glass plate on a plane parallel with the glass plate is termed as the Y-axis direction, and the direction orthogonal to the travelling direction of the glass plate as the X-axis direction.
Since the one-dimensional CCD cameras 3 of the conventional cut-edge flaw detecting apparatus are disposed in such a manner that the direction in which CCD-elements are arranged coincides with the X-axis direction, the apparatus can detect only cut-edge flaws present on the trimmer sides 5, i.e., two sides parallel with the Y-axis direction, but cannot detect those on the breaker sides 6, i.e., the two other sides parallel with the X-axis direction. Furthermore, this method, which also uses the CCD camera 3 to receive transmitted light has low accuracy in detecting burrs and notches for the same reasons as described above.
The conventional cut-edge flaw detecting device, which judges the size of a cut-edge flaw based on the magnitude of the differentiation signal obtained by differentiating the output of the CCD camera 3, has low accuracy in judging the size of flaw because the size of the cut-edge flaw is not necessarily proportional to the size of the differentiation signal.